In a paper manufacturing process which employs coated broke as a portion of the total pulp furnish, ironically charged relatively high molecular weight water soluble polymers are often employed to enhance retention of cellulosic fibers, fines, and inorganic fillers. The addition of these polymers produces a cleaner process stream by reducing the solids level in the process filtrate circuit. The high molecular weight polymers control solids by absorbing onto solid particulate surfaces in the papermaking furnish slurry and invoking charge neutralization (coagulation) and/or bridging (flucculation) mechanisms which cause the solid particles to flocculate. The flocculate can be retained by the formed mat of cellulosic fibers more easily than smaller individual particles. However, one disadvantage of the use of these polymers is that the flocculated particulate material can be transferred from the surface of the sheet to the paper machine press felts. I the flocculated state, the particles cannot pass through the fine, porous structure of the press felt and they become entrapped therein. If not controlled by adequate felt conditioning practices, these agglomerated particulate substances can severely impair the ability of the press felts to absorb water thereby requiring reduced production rates and shortening the useful life of the felts. In addition, it has been found that common polymeric retention aids can render normally effective prior art felt conditioners useless or marginally effective in part because of the size of the flocculated particles.
The use of polymeric retention aids is particularly critical for the efficient operation of neutral and alkaline paper making processes (pH approximately 6.0 to 8.5). Without the use of such polymeric retention aids, common system additives such as cellulose reactive sizes, alkenyl succinic anhydride (ASA) and alkene ketene diner (AKD), can cycle up in the process system and cause numerous operational problems, particularly in the press sections. Most of the particulates which are transferred from the sheet to the press felts in a flocculated state are too large to easily pass through the press felt. The contaminants thus become imbedded in the felt structure or are transferred back to the sheet and cause spots, holes or deposits in the dryer section.
Press felts associated with coated alkaline fine paper can experience excessive filling due to pitch deposits which arise from paper making furnish components such as sizing agents, alumina and fiber fines. Paper coating binders such as polyvinyl acetate or styrene butadiene lattices and inorganic coating pigments such as clay, calcium carbonate and titanium dioxide can also contribute to felt filling.
Analysis of used press felts from neutral and alkaline papermaking systems reveal a number of polymerically flocculated materials. Significant quantities of inorganic fillers such as; calcium carbonate, clay, and titanium dioxide alone or in association with particles of latex coating binders such as, polyvinyl acetate or styrene butadiene rubber are often found. Other types of contaminants that can be associated with the agglomerates, in significantly lower quantities can include starches, natural wood pitch (fatty esters, fatty acids and salts, resin acids and salts) cellulosic fiber fines, microbiological contaminants and absorbants, such as talc or bentonite, oil based defoamers and insoluble metal hydroxides. All of these contaminants can be present to some degree based on a variety of factors such as the pulp furnish and water sources, grade of material produced, type and quantity of system additives, pulp production methods and equipment design and capacities.
Processes to inhibit contaminating deposition in paper making felts are known in the art. U.S. Pat. No. 4,895,622 Barnett et al. discloses a process for press felt conditioning which controls the deposition of polymerically flocculated particulate substances by treating the press felts with a conditioner comprising a relatively low molecular weight organic, anionic polymer and at least one hydrophilic, nonionic or anionic surfactant. U.S. Pat. No. 3,398,047, Michalski, discloses a method of controlling pitch deposition in pulp and papermill systems by treating the system with a blend of a ligand and an organic sulfonate. U.S. Pat. No. 4,184,912, Payton discloses a method of preventing pitch formation by dispersing and emulsifying pitch particles in the pulp furnish to an exceptionally fine state and uniformly distributing the particles throughout the finished paper. The pitch deposition is controlled by the addition of a three component formulation comprising a nonionic surfactant plus an anionic surfactant and a low molecular weight anionic polymer. The three component mixture is added to the papermaking pulp system at a point prior to where pitch deposits normally form. In U.S. Pat. No. 3,992,249, Farley, a process for inhibiting pitch deposition is disclosed wherein the pulp is washed with an aqueous solution of anionic polymers having between 25 to 85 mole percent hydrophobic-oleophilic linkages and 15 to 70 mole percent hydrophilic acid linkages to complex with the pitch. The pitch-polymer complex is washed away with water.
U.S. Pat. No. 3,873,417, Otrhalek et al. discloses a pitch and pigment dispersant which comprises a neutralized solution of polymer prepared by free radical polymerization of an alpha, beta unsaturated acid with an alkyl ester and an allyl alcohol.